Pipelines of the type used to transport materials such as gas, oil, etc over long distances are formed of metallic pipeline sections. These are joined together with welds. In most situations, the pipelines are constructed by adding sections sequentially, one section after another. When a section has been added it is welded to the preceding section. Because the end of the newly added section is open, it is possible to inspect the weld from both sides, and this is conventionally done using x-ray penetration of the weld and a suitable detection system such as x-ray sensitive film. In some situations, internal access is difficult to obtain. For example, where a pipeline has to pass beneath a road, the pipeline may be constructed in conventional manner on either side of the road and subsequently a tunnel dug beneath the road through which a pipeline section can be passed which is then connected to the pipeline sections on either side. When this is done, it is often not possible to obtain internal access to the pipeline for inspection of the weld used to connect the pipeline sections. Therefore, a system of inspection completely external to the pipeline has to be provided.
Conventionally, such welds have been inspected by using a high strength, broad beam radioactive source such as x-ray or gamma-ray to penetrate both walls of the pipeline and to expose an x-ray/gamma-ray sensitive film plate on the opposite side of the pipeline to the x-ray/gamma-ray source.
In order to obtain a complete image of the weld a plurality of exposures are required and six or more is not uncommon, dependent upon pipeline diameter. This is to enable the weld to be inspected at all positions around the circumference of the pipeline. The broad beam nature of the source also requires the exposures to be relatively long and the radiation exclusion area which needs to be provided around the x-ray/gamma-ray source is therefore extensive.